This invention relates generally to a method of driving the heat-sensitive head of a heat-sensitive recording apparatus and, more particularly, to such a method which increases the speed of thermal recording without reducing the quality and density of the recording or printing circuits.
In such a heat-sensitive recording apparatus, video signals for each scanning line which are supplied from a reading unit are employed to drive its heat-sensitive recording head to selectively cause the heat generating resistors to generate heat, so that the video signals are thermally recorded on a heat-sensitive recording sheet which is moved in the auxiliary scanning direction while being in contact with the heat generating resistors. The following patents and allowed patent application disclose heat-sensitive recording heads and apparatuses of this general type, and their disclosures are expressly incorporated herein by reference: U.S. Pat. Nos. 3,934,695, 4,032,925, 4,091,391, 4,113,391, 4,168,421 and 4,219,824; Ser. No. 127,690, filed Mar. 6, 1980.
FIG. 1 diagrammatically shows the recording section of a heat-sensitive recording device, which section drives a heat-sensitive recording head (not shown) with video signals set or stored in a shift register 1. The shift register 1 performs serial-to-parallel signal conversion. Serial video signals for one scanning line are supplied to the input 2 of shift register 1 during a set time interval T.sub.0 extending from the time instant t.sub.0 to the time instant t.sub.1 as shown in FIG. 2A, and are set therein. The video signals thus set are applied, as video signals corresponding to picture elements, to respective lead wires 3 connected to the outputs of shift register 1. First ends of these lead wires 3 are electrically connected to a heat generating resistor 4 at predetermined intervals. First ends of lead wires 5 are also electrically connected to the heat generating resistor 4 at the same intervals as those mentioned above. However, it should be noted that the lead wires 3 and the lead wires 5 are staggered as shown in FIG. 1 so that the lead wires 3 and 5 do not overlap. Therefore, when a switching element (not shown) connected to the other ends of the lead wires 5 is turned on during the printing time interval T.sub.1 extending from the time instant t.sub.1, when the video signals 3 are set in the shift register 1, to the predetermined time instant t.sub.2, earlier than the time instant t.sub.3 when the next video signals are set in the shift register 1 (FIG. 2B), then a pair of lead wires 3 and 5 corresponding to a "black" video signal are supplied with current, as a result of which the corresponding portion of the heat generating resistor 4 generates heat to achieve the thermal recording operation.
During the time interval T.sub.2 extending from the time instant t.sub.2, when the recording or printing has been accomplished, to the time instant t.sub.3, when video signals for the next line are set in the shift register, the heat-sensitive recording sheet (not shown) must be moved in the auxiliary scanning direction to the next line printing or recording position. Accordingly, it is impossible to reduce the time interval T.sub.2 to any great extent. However, if the recording speed of the heat-sensitive recording apparatus is increased, i.e. if the recording time (T.sub.0 +T.sub.1 +T.sub.2) per line is reduced, then the printing time is also reduced. On the other hand, as the printing time is reduced, the amount of heat generated by each selected portion of the heat generating resistor 4 is also reduced. In order to compensate for this reduction in heat, a method of increasing the voltage applied to the heat-sensitive recording head has been employed in the art. However, the increase of the voltage is limited to a certain maximum value because of the durability of the heat-sensitive recording head and the design of the heat-sensitive recording head drive circuit, and, therefore, it is difficult for such a high speed heat-sensitive recording apparatus to produce a thermal recording with sufficiently high density and quality.